目的:使用蒙特卡罗(MC)结合体素模型分析(125)I前列腺植入物的剂量分布和等效均匀剂量(EUD)中的组织异质性效应。
背景:低剂量率近距离放射治疗中的剂量分布计算是基于水模中单个源周围的剂量沉积。这种形式主义没有考虑到组织的异质性,种子间衰减,或有限的患者尺寸的影响。由于光电效应,组织组成尤为重要。
方法:使用两名前列腺癌患者的计算机断层摄影(CT)来创建用于MC模拟的体素模型。将元素组成和密度分配给每个结构。前列腺的密度,囊泡,通过100例患者的CT电子密度确定直肠和膀胱。考虑到与纯水相同的体模,进行相同的模拟。通过前列腺和直肠的剂量-体积直方图和EUD比较结果。
结果:前列腺的平均吸收剂量偏差为3.3-4.0%,直肠的平均吸收剂量偏差为2.3-4.9%。当比较水中的计算与异质体模中的计算时。在水中的计算中,前列腺D90被高估2.8-3.9%,直肠D0.1cc导致6-8%的剂量差异.EUD导致前列腺的高估为3.5-3.7%,直肠的高估为7.7-8.3%。
结论:对于水中的模拟,沉积剂量始终被高估。为了提高确定剂量分布的准确性,尤其是在直肠周围,建议引入基于模型的算法。
OBJECTIVE: To use Monte Carlo (MC) together with voxel phantoms to analyze the tissue heterogeneity effect in the dose distributions and equivalent uniform dose (EUD) for (125)I prostate implants.
BACKGROUND: Dose distribution calculations in low dose-rate brachytherapy are based on the dose deposition around a single source in a water phantom. This formalism does not take into account tissue heterogeneities, interseed attenuation, or finite patient dimensions effects. Tissue composition is especially important due to the photoelectric effect.
METHODS: The computed tomographies (CT) of two patients with prostate cancer were used to create voxel phantoms for the MC simulations. An elemental composition and density were assigned to each structure. Densities of the prostate, vesicles, rectum and bladder were determined through the CT electronic densities of 100 patients. The same simulations were performed considering the same phantom as pure water. Results were compared via dose-volume histograms and EUD for the prostate and rectum.
RESULTS: The mean absorbed doses presented deviations of 3.3-4.0% for the prostate and of 2.3-4.9% for the rectum, when comparing calculations in water with calculations in the heterogeneous phantom. In the calculations in water, the prostate D 90 was overestimated by 2.8-3.9% and the rectum D 0.1cc resulted in dose differences of 6-8%. The EUD resulted in an overestimation of 3.5-3.7% for the prostate and of 7.7-8.3% for the rectum.
CONCLUSIONS: The deposited dose was consistently overestimated for the simulation in water. In order to increase the accuracy in the determination of dose distributions, especially around the rectum, the introduction of the model-based algorithms is recommended.